On construction job sites, there typically is a need to readily determine true elevations and/or relative elevations at specific locations. For example, it often is necessary to determine how much material should be added or removed at specific locations to create a level surface for a building foundation. Conventionally, an optical instrument (e.g., a level, a transit, or a theodolite) and a grade rod are used in conjunction to generate these measurements. On most modern construction job sites, the optical instrument is a laser that projects a level laser beam. The laser is placed at a fixed location and the grade rod is moved throughout the job site to take height measurements at specific locations.
Most conventional grade rods are elongated poles with fixed height markings in metric and/or English units. For example, such grade rods can include a scale from zero feet (at the base) to six feet (near the top), with markings for hundredths of a foot (or markings for inches and fractions of inches) between the markings for feet. The laser beam can be either visibly projected onto the grade rod or received by a laser detector mounted on the grade rod. The markings on the grade rod at the point onto which the laser beam is projected indicate the distance between the ground (or other object) at the measurement location and the elevation of the laser. For example, if the laser is at a known elevation of 255 feet above sea level and the laser beam is detected at a height of 5 feet on the grade rod, the elevation at the measurement location is 255−5=250 feet above sea level.
Using grade rods with fixed height markings to gather true elevation measurements requires field calculations. Specifically, it is necessary to subtract the measurement at the point onto which the laser beam is projected from the known elevation of the laser. Once the true elevation at the measurement location has been determined, it often is necessary to subtract this value from a target value to determine how much material should be added (i.e., filled) or removed (i.e., cut) at the measurement location. For example, if the true elevation at the measurement location is 250 feet above sea level and the target for the measurement location is 253 feet above sea level, 253−250=3, so 3 feet of material should be added. If the result of this calculation is negative, it indicates the amount of material that should be removed from the measurement location. When dozens of measurements must be taken and the measurements include inches and fractions of inches, these field calculations can become burdensome and complex, which can lead to costly errors.
Grade rods with adjustable displays (some varieties of which are referred to as “direct reading” grade rods) were developed to reduce the need for field calculations. Such grade rods are described, for example, in U.S. Pat. No. 4,471,532. These grade rods typically include a measurement tape that can be adjusted so that the positions of the measurement markings move relative to the overall grade rod. For example, the measurement tape can encircle a portion of the grade rod and be rotatable around that portion of the grade rod. This allows the portion of the measurement tape within a display region on one side of the grade rod to be shifted. To determine true elevations, the grade rod can be placed on a location with a known elevation. A laser detector mounted on the grade rod then can be moved up or down until it detects a projected laser beam. Next, the measurement tape can be moved so that the measurement adjacent to a measurement indicator coupled to the laser detector corresponds to the last whole foot digit, inches, and fractions of inches of the known elevation. For example, if the known elevation is 255 feet 4¼ inches above seal level, the measurement tape can be shifted so that the measurement adjacent to the measurement indicator is 5 feet 4¼ inches. Once the position of the measurement tape has been set and locked in place, the overall grade rod can be moved to different locations. At each location, the laser detector can be moved up or down until it detects the projected laser beam. The measurement adjacent to the measurement indicator then indicates the last whole foot digit, inches, and fractions of inches of the true elevation at the measurement location. The remaining digits can be committed to memory or written in a convenient location.
Grade rods with adjustable displays also can be used to conveniently determine how much material should be added or removed at a measurement location. The procedure is similar to the procedure described above. At a reference location (e.g., the planned finish grade for a job site), the measurement tape can be adjusted until a zero marking on a cut/fill scale is aligned with the measurement indicator coupled to the laser detector. The cut/fill scale can include measurements extending above and below the zero marking. The grade rod then can be moved to different locations. At each location, the laser detector can be moved up or down until it detects the projected laser beam. If, at a measurement location, the measurement indicator is above the zero marking, the measurement adjacent to the measurement indicator indicates the amount of fill necessary to bring the grade at the measurement location up to the reference grade. Similarly, if the measurement indicator is below the zero marking, the measurement adjacent to the measurement indicator indicates the amount of cut necessary to bring the grade at the measurement location down to the reference grade.